An exhaust gas purifier suitable for use in a diesel engine of the type in which the intake air is throttle by a throttle valve provided in an intake passage during engine idling to reduce engine vibration and noise. Unburned combustibles produced by intake air throttling are treated by the exhaust gas purifier having a main passage and a bypass passage in which an oxidation catalyst is received. A bypass control valve, operable in synchronization with the throttle valve, urges the exhaust gas to flow through the bypass passage during engine idling. Preferably, means for preheating the catalyst is provided, which comprises an injector and an igniting glow plug.